Material processing using plasma is a well-known technique. Processing is performed on a workpiece such as glass plates, semiconductor wafers or metal substrates. The processing may involve modification of a surface of the workpiece using the plasma, deposition of a thin film on the surface, and/or etching the film with the plasma. Gas plasma is formed through electrical excitation of a gas to produce electron-ion pairs.
In general, systems for material processing with plasma typically include a plasma chamber containing, or adjacent to, a plasma source powered by a radio frequency (RF) generator. In this type of system, the workpiece and the gas are placed in the plasma chamber and power is supplied to the gas by the RF generator via an antenna or electrode. A typical plasma generator is an RF power supply operating at an RF frequency of, for example, 100 kHz-100 MHz and an application-specific power level. RF power supplied by the RF generator excites the gas to form a plasma load.
During operation, the RF generator continuously delivers RF power. The RF power may be modulated in an attempt to sustain the plasma load in a discharge state by maintaining an equilibrium operating point. Modulation of the delivered RF power, however, may modulate the plasma load but not necessarily in a linear fashion. Accordingly, modulation of the plasma load is usually not desirable (unless operating in a pulsed mode) but may arise as a result of instabilities in the RF generator, the plasma load or the combined response of the RF generator and the plasma load.
Some instabilities of the plasma may be a result of the impedance of the plasma load. The plasma load impedance may interact with the dynamic output impedance of the RF generator at one or more frequencies to produce unstable positive loop gain. The unstable positive loop gain has the effect of allowing minor perturbations from the equilibrium operating point to significantly affect the RF power supplied by the RF generator. As a result, plasma extinguishment or oscillatory behavior of the plasma may occur. Unstable positive loop gain is typically easier to control in an RF generator that includes a linear power amplifier as the output stage, due, in part, to the wide modulation bandwidth possible. Conversely, in RF generators that include a nonlinear switching power amplifier as the output stage, the complex pole-zero response restricts the available modulation bandwidth and makes control more difficult.
In prior art systems, well-known techniques that may provide some suppression of such instabilities include modification of the length of a coaxial cable connecting the RF generator and the plasma chamber and/or impedance matching networks with fixed or variable impedance. Another prior art technique involves the use of a power sensor. The power sensor is employed in a feedback control scheme to control the RF output power of an RF generator that includes a switch-mode power amplifier output stage. Such a feedback control scheme may operate with a frequency response up to about 10 kHz, although 1 or 2 kHz is more typical. Through control of the RF output power, nascent instabilities of the plasma impedance may be dominated to ensure the effective gain of any oscillation of the plasma impedance is below unity. By maintaining oscillations below unity gain, a metastable operating point may be formed around the equilibrium operating point.
Known problems in the prior art techniques and systems involve the inability to track and address instabilities occurring at frequencies higher than the modulation bandwidth of the RF generator. This is especially true when a switching power amplifier is used as the output stage of the RF generator. In general, the more efficient the power amplifier, the more opportunity for unstable operation with unmatched impedance and/or highly non-linear loads such as the plasma load.
Prior art systems and techniques for controlling instabilities lack sufficiently fast frequency response to react quickly enough to high frequency changes in the impedance of the plasma load. Impedance matching networks are limited to the speed at which the variable impedance components (or the frequency of the RF generator) may be adjusted. Similarly, feedback control schemes utilizing a power sensor are unable to track and react quickly enough to high frequency changes in the RF power. Often, the control response bandwidth may be directly limited by the rate at which the RF output power may be modulated. In fact, attempting feedback control near the bandwidth limit of the RF generator with prior art techniques may increase instability due to strong phase shifts in the output phase of the RF power near the bandwidth limit frequency. As such, changes in the impedance of the plasma load using delivered RF power may create instability that causes undesirable oscillations and/or extinguishments near the bandwidth limit.